Int 21H, AH=4AH

2 and higher

RESIZE MEMORY BLOCK

AH = 4Ah
BX = new size in paragraphs
ES = segment of block to resize

 CF clear if successful
CF set on error
    AX = error code (07h,08h,09h) (see #01680 at AH=59h/BX=0000h)
    BX = maximum paragraphs available for specified memory block

under DOS 2.1-6.0, if there is insufficient memory to expand the block
  as much as requested, the block will be made as large as possible
DOS 2.1-6.0 coalesces any free blocks immediately following the block
  to be resized
        

AH=48h,AH=49h,AH=83h

Text based on Ralf Brown Interrupt List Release 61